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United States Patent |
5,573,843
|
Mitoh
,   et al.
|
November 12, 1996
|
Fiber-reinforced plastic composite material for fastening pallets and
process for producing the same
Abstract
Disclosed herein is a fiber-reinforced plastic composite material for
fastening pallets to be used to transfer printed circuit boards for
automatic soldering. The composite material is composed of a thermosetting
resin, reinforcing long fibers, and a light-colored electrically
conductive filler. The pallets are formed from the composite material by
heating and pressing. The pallets have high electrical conductivity to
prevent static build-up, a light color which permits coloring, and good
mechanical properties.
Inventors:
|
Mitoh; Yutaka (Kobe, JP);
Fujiwara; Noaya (Kobe, JP)
|
Assignee:
|
Kabushiki Kaisha Kobe Seiko Sho (Kobe, JP)
|
Appl. No.:
|
279844 |
Filed:
|
July 26, 1994 |
Foreign Application Priority Data
Current U.S. Class: |
428/300.7; 428/327; 428/328; 428/330; 428/931 |
Intern'l Class: |
B32B 005/16 |
Field of Search: |
428/327,224,232,240,242,245,931,327,328,330
|
References Cited
U.S. Patent Documents
4944998 | Jul., 1990 | Ko et al. | 428/327.
|
Primary Examiner: Ryan; Patrick
Assistant Examiner: Weisberger; Rich
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier, Neustadt, P.C.
Claims
What is claimed is:
1. A solder resistant fastening pallet suitable to transfer printed circuit
boards, comprising a fiber-reinforced plastic composite material, said
composition material having a surface resistivity of from about
2.times.10.sup.5 to 8.times.10.sup.5, a flexural strength of from about
29.9 to 37.7 kg/mm.sup.2, and a flexural modulus of from about 1,722 to
1,966 kg/mm.sup.2, and being a thermosetting resin having incorporated
therein 50-80 wt % reinforcing fibers and 5-90 parts by weight of an
electrically conductive filler selected from the group consisting of
K.sub.2 Ti.sub.8 O.sub.17, SnO.sub.2, Sb.sub.2 O.sub.3, Sb.sub.2 O.sub.5,
ZnO, TiO.sub.2 and mixtures thereof.
2. The fastening pallet as defined in claim 1, wherein the thermosetting
resin is an epoxy resin which is liquid at room temperature before curing
and has a glass transition point higher than 150.degree. C. after curing.
3. A process for producing a fastening pallet as defined in claim 1, said
process comprising impregnating a laminate of reinforcing fibers with a
thermosetting resin liquid in which is dispersed said electrically
conductive filler, followed by heating and pressing.
4. A process for producing a fiber-reinforced plastic composite material
for fastening pallets as defined in claim 3, wherein the thermosetting
resin is an epoxy resin which is liquid at room temperature before curing
and has a glass transition point higher than 150.degree. C. after curing.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fiber-reinforced plastic composite
material for fastening pallets to be used to transfer printed circuit
boards and also to a process for producing the same. The composite
material has high mechanical strength, good antistatic property, good heat
resistance, and good machinability. It can be colored in any light color.
2. Description of the Prior Art
The mounting of semiconductor parts on a printed circuit board is usually
accomplished by means of an automatic soldering machine. For semiconductor
parts to be placed at right soldering positions, it is necessary to carry
the printed circuit board to a desired position in the machine, to stop it
there, and fix it there accurately, without causing warpage. A known way
to accomplish this is to transfer a printed circuit board by the aid of a
pallet to which it is fastened. (See Japanese Patent Publication No.
20786/1980.)
This type of fastening pallet for transfer is usually made of a metal frame
(of stainless steel, aluminum, or titanium) and a means to secure a
printed circuit board. Being of general-purpose type, it can be applied to
a large variety of printed circuit boards but, on the other hand, it needs
fine adjustment for fastening position according to the specific printed
circuit board to be transferred. This adjustment is troublesome and
inefficient. In addition, the recent trend toward the miniaturization and
weight reduction of electronic machines and equipment has evoked the need
for printed circuit boards of varied shapes and for flexible printed
circuit boards. Under these circumstances, it turned out the conventional
general-purpose fastening pallets do not meet requirements for efficient
operation.
To cope with this situation, there has been proposed a new fastening pallet
that can be applied to flexible printed circuit boards efficiently. It is
made of metal plate formed in conformity with the configuration of a
particular printed circuit board. (See Japanese Patent Laid-open No.
54991/1990.) However, this pallet suffers the disadvantage of being heavy
and having a high thermal conductivity (because it is made of metal).
Thus, it easily gets hot during soldering, which causes the sticking of
solder.
To solve this problem, there has been proposed a fastening pallet of
special-purpose type which is made of fiber-reinforced plastic laminate
formed in conformity with a particular printed circuit board. This
laminate is prepared by heating and pressing from a half-cured prepreg of
nonwoven fabric of glass fiber impregnated with an epoxy resin. The
fastening pallet of this kind, however, has the disadvantage that the
composite resin is subject to deterioration due to heat to which it is
exposed repeatedly during its use and this deterioration leads to
delamination.
Moreover, the fastening pallet made of plastic composite material is liable
to accumulate static charge during soldering, and there is an extreme case
in which the accumulated static charge breaks electronic parts on the
printed circuit board. To avoid this trouble, an attempt has been made to
incorporate the composite material with carbon black, thereby rendering
the composite material electrically conductive (about 10.sup.7
.OMEGA./.quadrature.) and permitting the leakage of static charge.
Unfortunately, carbon black incorporated into the plastic composite
material falls off or fouls electronic parts (resulting in
short-circuiting) when rubbed during use. In addition, carbon black makes
the pallet black and the black color brings about heat loss at the time of
heating and prevent the coloring for good appearance.
There is another way of rendering the plastic composite material
electrically conductive by incorporating it with metal fiber or metal
powder. (See Japanese Patent Laid-open No. 31346/1991.) Metal fiber does
not disperse uniformly when the composite material undergoes laminate
forming. This leads to uneven electrical conductivity. Metal powder is
liable to precipitate (due to its higher specific gravity) and hence does
not distribute uniformly. This also leads to uneven electrical
conductivity. Moreover, metal powder involves the danger of dust explosion
in the manufacturing process.
SUMMARY OF THE INVENTION
The present invention was completed to address the above-mentioned
problems. It is an object of the present invention to provide a
fiber-reinforced plastic composite material for fastening pallets which
have uniform electrical conductivity, good heat resistance, good
mechanical properties, and good resistance to delamination. The composite
material itself has a light color and is capable of being colored and
formed as desired.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The gist of the present invention resides in a fiber-reinforced plastic
composite material for fastening pallets to be used to transfer printed
circuit boards, said composite material being a thermosetting resin
incorporated with reinforcing long fibers and a light-colored electrically
conductive filler. The plastic composite material is produced by
impregnating a laminate of reinforcing long fibers with a thermosetting
resin liquid in which is dispersed a light-colored electrically conductive
filler, followed by heating and pressing.
According to the present invention, the plastic composite material is
composed of reinforcing long fibers and a thermosetting resin in which is
uniformly dispersed a light-colored electrically conductive filler. The
long fibers are responsible for high strength and the uniformly dispersed
filler is responsible for uniform conductivity and antistatic property.
Being held in the interstices of the long fibers, the filler does not fall
off due to rubbing. The filler imparts good heat resistance to the
composite material. The light-colored filler permits the composite
material to be colored with a pigment or dye of desired color. The
composite material is capable of machining into any shape according to
printed circuit boards to be transferred. The fastening pallets can be
adapted to printed circuit boards of any shape for their transfer and
fixing.
The reinforcing long fibers used in the present invention are not
specifically limited in their kind. Their preferred examples include
inorganic long fibers (such as glass fibers) and heat-resistant organic
long fibers (such as aramid fibers). They may be used alone or in
combination with one another. They may be used in the form of nonwoven
fabric, woven fabric, or knitted fabric.
The light-colored electrically conductive filler is not specifically
limited in kind. It includes, for example, zinc oxide, titanium oxide, and
potassium titanate whisker or powder. They are commercially available
under the trade name of "Pastran" (from Mitsui Mining and Smelting Co.,
Ltd.), "Dentol" (from Otsuka Chemical Co., Ltd.), "Panatetra" (from
Matsushita Amtec Co., Ltd.), "W-1-P" and "W-1" (from Mitsubishi Material
Co., Ltd.), and "Taipaig FT-1000" and "Taipaig ET-500W" (from Ishihara
Sangyo Kaisha, Ltd.). They may be used alone or in combination with one
another. They may also be used in combination with a non-conductive filler
(such as talc, kaolin, mica, glass flake, and titanium oxide) in an amount
not harmful to electrical conductivity, to facilitate impregnation and
workability.
The light-colored electrically conductive filler should preferably be finer
than 20 .mu.m in diameter so that it is dispersed uniformly into the
resin. The filler is not specifically limited in shape; however, an
elongated shape (with a great aspect ratio) is preferable to a spherical
shape from the standpoint of ability to impart electrical conductivity.
According to the present invention, the matrix resin should be a
thermosetting resin which is a low-viscosity liquid before curing (so that
it is easily impregnated into reinforcing long fibers) and becomes solid
by curing upon heating and pressing after impregnation. The matrix resin
should have sufficient heat resistance so that the fastening pallets made
from it are not deformed by heat of molten solder with which they come
into direct contact. A preferred matrix resin is one which has a glass
transition point higher than 150.degree. C. after curing. The formulation
of the matrix resin should be established so that these conditions are
met. For example, the matrix resin may be composed of an epoxy resin and
an acid anhydride hardener. The epoxy resin includes bisphenol A type
epoxy resin, bisphenol F type epoxy resin, bisphenol S type epoxy resin,
phenol novolak type epoxy resin, polyfunctional glycidylamine type epoxy
resin, polyfunctional glycidylether type epoxy resin, and alicyclic epoxy
resin. The hardener includes methylnadic acid anhydride,
methylhexahydrophthalic anhydride, and methyltetrahydrophthalic anhydride.
They may be used alone or in combination with one another.
Preferable among these examples are bisphenol A type epoxy resin (or phenol
novolak type epoxy resin) and methylnadic acid anhydride. The hardener may
be used in combination with a proper amount of cure accelerator such as
dimethylbenzineamine, 2-ethyl-4-methylimidazole, and
1-butyl-2-methylimidazole.
The amount of the reinforcing long fibers in the plastic composite material
should be 50-80 wt %, preferably 60-75 wt %. An amount less than 50 wt %
is not enough for sufficient reinforcement; and an amount in excess of 80
wt % leads to poor impregnation and a shortage of resin to hold the fibers
and filler.
The adequate amount of the electrically conductive filler varies depending
on its kind, shape, and particle diameter. It is usually 5-90 parts by
weight, preferably 10-50 parts by weight, for 100 parts by weight of the
resin, so that the filler imparts uniform, stable electrical conductivity
to the composite material without adverse effect on impregnation and any
possibility of falling off. It was found that the adequate amount of
commercially available conductive fillers is as follows:
"Dentol" . . . about 10 parts by weight
"Panatetra" . . . about 30 parts by weight
"W-1-P" . . . about 90 parts by weight
The plastic composite material of the present invention is prepared by
impregnating a laminate of mats (in the form of nonwoven or woven fabric
of reinforcing long fibers) with a thermosetting resin liquid in which is
dispersed an adequate amount of the electrically conductive filler, and
then pressing and heating the impregnated laminate into a desired shape.
During impregnation, the filler is evenly dispersed into the interstices
of the reinforcing long fibers without the possibility of settling due to
difference in specific gravity. Thus there is obtained the desired
composite material which has uniform physical properties and electrical
conductivity.
The thus obtained plastic composite material is composed of a thermosetting
resin matrix, particles of electrically conductive filler dispersed in the
resin matrix, and reinforcing long fibers impregnated with the resin
matrix. It has high strength as well as uniform electrical conductivity.
It does not permit the filler to fall off when rubbed. It is not subject
to delamination. It can be easily machined into a desired shape in
conformity to the shape and structure of the printed circuit board to be
transferred. Being made of a light-colored material, it suffers less heat
loss than the black material at the time of heating and it can be colored
in any desired color (for aesthetic purpose) by incorporation with an
adequate colorant.
EXAMPLES
The following examples are included merely to aid in the understanding of
the invention, and variations may be made by one skilled in the art
without departing from the spirit and scope of the invention. Unless
otherwise stated, quantities are expressed as parts by weight.
The performance of the molded product was evaluated according to the
following methods.
Glass transition point: measured using a differential scanning calorimeter,
Model DSC7 (Perkin-Elmer), with the temperature increasing at a rate of
10.degree. C./min.
Electrical conductivity: evaluated in terms of surface resistivity measured
using "Hirester IP" made by Mitsubishi Petrochemical Co., Ltd.
Heat resistance: by visual observation after immersion in a soldering bath
at 300.degree. C. for 10 minutes. Rated as "good" (no change) and "poor"
(delamination due to thermal decomposition).
Color: visual observation.
EXAMPLE 1
A resin liquid was prepared by uniform mixing from the following
components.
Epoxy resin ("DER331" from Dow Chemical) . . . 75 parts
Epoxy resin ("Tetrad-C" from Mitsubishi Gas Chemical) . . . 25 parts
Acid anhydride hardener ("Kayahard MCD" from Nippon Kayaku) . . . 105 parts
Cure accelerator ("2E4MZ" (2-ethyl-4-methylimidazole) from Shikoku Kasei) .
. . 1 part
Electrically conductive filler ("Dentol WK-208" from Otsuka Chemical) . . .
10 parts
Titanium oxide . . . 5 parts
The resin liquid was impregnated into a laminate of nonwoven fabrics of
glass fiber, with the ratio of the former to the latter being 20:80 by
weight. The impregnated laminate was pressed at 160.degree. C. for 10
minutes using a hot press to give a molded product. Upon curing at
160.degree. C. for 10 minutes, the resin composition used in this example
gave a cured product having a glass transition point of 185.degree. C.,
which is an indication of sufficient heat resistance required of the
matrix resin for fastening pallets.
The resulting molded product assumed a light grayish color. It had a
surface resistivity of 2.times.10.sup.5 .OMEGA./.quadrature., indicating
that it has electrical conductivity high enough to prevent static
build-up. Flexural tests indicated that the molded product has a flexural
strength of 37.7 kg/mm.sup.2 and a flexural modulus of 1966 kg/mm.sup.2. A
test piece (5 cm square) cut out of the molded product remained intact
without delamination when immersed in a soldering bath at 300.degree. C.
for 10 minutes. The molded product was fabricated into a fastening pallet
by machining in conformity with a printed circuit board, and the pallet
was used for the automatic soldering of the printed circuit board. It was
possible to carry out satisfactory soldering.
EXAMPLE 2
The resin liquid prepared in Example 1 was further incorporated with 30
parts of "Panatetra" and 5 parts of titanium oxide as electrically
conductive fillers. The resin liquid was impregnated into a laminate of
nonwoven fabrics of glass fiber, with the ratio of the former to the
latter being 30:70 by weight. The impregnated laminate was pressed at
160.degree. C. for 10 minutes using a hot press to give a molded product.
The molded product assumed a white color. It had a surface resistivity of
8.times.10.sup.5 .OMEGA./.quadrature., indicating that it has electrical
conductivity high enough to prevent static build-up. Flexural tests
indicated that the molded product has a flexural strength of 33.8
kg/mm.sup.2 and a flexural modulus of 1850 kg/mm.sup.2. A test piece (5 cm
square) cut out of the molded product remained intact without delamination
(due to thermal decomposition) when immersed in a soldering bath at
300.degree. C. for 10 minutes.
EXAMPLE 3
The resin liquid prepared in Example 1 was further incorporated with 90
parts of "W-1-P" as an electrically conductive filler. The resin liquid
was impregnated into a laminate of nonwoven fabrics of glass fiber, with
the ratio of the former to the latter being 40:60 by weight. The
impregnated laminate was pressed at 160.degree. C. for 10 minutes using a
hot press to give a molded product. The molded product assumed a blue
color. It had a surface resistivity of 8.times.10.sup.5
.OMEGA./.quadrature., indicating that it has electrical conductivity high
enough to prevent static build-up. Flexural tests indicated that the
molded product has a flexural strength of 29.9 kg/mm.sup.2 and a flexural
modulus of 1722 kg/mm.sup.2. A test piece (5 cm square) cut out of the
molded product remained intact without delamination (due to thermal
decomposition) when immersed in a soldering bath at 300.degree. C. for 10
minutes.
COMPARATIVE EXAMPLE 1
The resin liquid prepared in Example 1 was further incorporated with 30
parts of carbon black and 20 parts of titanium oxide as electrically
conductive fillers. The resin liquid was impregnated into a laminate of
nonwoven fabrics of glass fiber, with the ratio of the former to the
latter being 30:70 by weight. The impregnated laminate was pressed at
160.degree. C. for 10 minutes using a hot press to give a molded product.
The molded product assumed a black color. It had a surface resistivity of
1.times.10.sup.5 .OMEGA./.quadrature., indicating that it has electrical
conductivity high enough to prevent static build-up. Flexural tests
indicated that the molded product has a flexural strength of 34.7
kg/mm.sup.2 and a flexural modulus of 1866 kg/mm.sup.2. A test piece (5 cm
square) cut out of the molded product remained intact without delamination
(due to thermal decomposition) when immersed in a soldering bath at
300.degree. C. for 10 minutes.
COMPARATIVE EXAMPLE 2
The resin liquid prepared in Example 1 was further incorporated with 1 part
of carbon black and 30 parts of titanium oxide as electrically conductive
fillers. The resin liquid was impregnated into a laminate of nonwoven
fabrics of glass fiber, with the ratio of the former to the latter being
40:60 by weight. The impregnated laminate was pressed at 160.degree. C.
for 10 minutes using a hot press to give a molded product. The molded
product assumed a blackish gray color (which is more desirable than the
color in Comparative Example 1). It had a surface resistivity of
7.times.10.sup.8 .OMEGA./.quadrature., indicating that it does not have
electrical conductivity high enough to prevent static build-up. Flexural
tests indicated that the molded product has a flexural strength of 24.6
kg/mm.sup.2 and a flexural modulus of 1480 kg/mm.sup.2. A test piece (5 cm
square) cut out of the molded product remained intact without delamination
(due to thermal decomposition) when immersed in a soldering bath at
300.degree. C. for 10 minutes.
COMPARATIVE EXAMPLE 3
The resin liquid prepared in Example 1 was further incorporated with 5
parts of iron powder as an electrically conductive filler. The resin
liquid was impregnated into a laminate of nonwoven fabrics of glass fiber,
with the ratio of the former to the latter being 30:70 by weight. The
impregnated laminate was pressed at 160.degree. C. for 10 minutes using a
hot press to give a molded product. The molded product assumed a yellow
color (originating from the color of the epoxy resin). However, it was
translucent, permitting the uneven distribution of iron powder to be
observed. It had a surface resistivity of 5.times.10.sup.12
.OMEGA./.quadrature., indicating that it does not have electrical
conductivity high enough to prevent static build-up. Flexural tests
indicated that the molded product has a flexural strength of 32.4
kg/mm.sup.2 and a flexural modulus of 1948 kg/mm.sup.2. A test piece (5 cm
square) cut out of the molded product remained intact without delamination
(due to thermal decomposition) when immersed in a soldering bath at
300.degree. C. for 10 minutes.
COMPARATIVE EXAMPLE 4
The resin liquid prepared in Example 1 was further incorporated with 20
parts of "Dentol WK-208" and 5 parts of titanium oxide as electrically
conductive fillers. The resin liquid was impregnated into a nonwoven
fabric of glass fiber, with the ratio of the former to the latter being
30:70 by weight. The impregnated nonwoven fabric was pressed at
100.degree. C. for 5 minutes using a hot press to give a prepreg in the B
stage. A laminate of the prepregs was pressed 160.degree. C. for 10
minutes to give a molded product. The molded product assumed a light
grayish color. It had a surface resistivity of 2.times.10.sup.7
.OMEGA./.quadrature., indicating that it does not have electrical
conductivity high enough to prevent static build-up. Flexural tests
indicated that the molded product has a flexural strength of 30.9
kg/mm.sup.2 and a flexural modulus of 1771 kg/mm.sup.2. A test piece (5 cm
square) cut out of the molded product suffered partial delamination near
the edges when immersed in a soldering bath at 300.degree. C. for 10
minutes.
COMPARATIVE EXAMPLE 5
The resin liquid prepared in Example 1 was further incorporated with 20
parts of "Dentol WK-208" and 5 parts of titanium oxide as electrically
conductive fillers. The resin liquid was impregnated into a nonwoven
fabric of glass fiber, with the ratio of the former to the latter being
15:85 by weight. The impregnated nonwoven fabric was pressed at
160.degree. C. for 10 minutes using a hot press to give a molded product.
The molded product was unsatisfactory due to incomplete impregnation near
the edges. The well-impregnated central part of the molded product assumed
a light gray color. The central part had a surface resistivity of
2.times.10.sup.5 .OMEGA./.quadrature., indicating that it has electrical
conductivity high enough to prevent static build-up. Flexural tests
indicated that the molded product has a flexural strength of 30.6
kg/mm.sup.2 and a flexural modulus of 1780 kg/mm.sup.2. A test piece (5 cm
square) cut out of the molded product remained intact without delamination
(due to thermal decomposition) when immersed in a soldering bath at
300.degree. C. for 10 minutes.
COMPARATIVE EXAMPLE 6
A resin liquid was prepared by uniform mixing from the following
components.
Epoxy resin ("DER331") . . . 100 parts
Acid anhydride hardener ("MT-500" from Nippon Kayaku) . . . 87 parts
Cure accelerator ("2E4MZ") . . . 1 part
Electrically conductive filler ("Dentol WK-208") . . . 10 parts
Titanium oxide . . . 5 parts
The resin liquid was impregnated into a laminate of nonwoven fabrics of
glass fiber, with the ratio of the former to the latter being 20:80 by
weight. The impregnated laminate was pressed at 160.degree. C. for 10
minutes using a hot press to give a molded product. Upon curing at
160.degree. C. for 10 minutes, the resin composition used in this example
gave a cured product having a glass transition point of 137.degree. C.
The resulting molded product assumed a light grayish color. It had a
surface resistivity of 2.times.10.sup.5 .OMEGA./.quadrature., indicating
that it has electrical conductivity high enough to prevent static
build-up. Flexural tests indicated that the molded product has a flexural
strength of 35.5 kg/mm.sup.2 and a flexural modulus of 1945 kg/mm.sup.2. A
test piece (5 cm square) cut out of the molded product remained intact
without delamination (due to thermal decomposition) when immersed in a
soldering bath at 300.degree. C. for 10 minutes. The molded product was
fabricated into a fastening pallet by machining in conformity with a
printed circuit board, and the pallet was used for the automatic soldering
of the printed circuit board. The printed circuit board suffered
incomplete soldering at its center which is presumably due to the thermal
deformation of the fastening pallet.
The results of Examples 1 to 3 and Comparative Examples 1 to 6 are
collectively shown in Table 1.
TABLE 1
__________________________________________________________________________
Example
Surface Flexural
Flexural
Performance
(Comparative
resistivity
Heating
strength
modulus
of fastening
Example)
(.OMEGA./.quadrature.)
Color
test (kg/mm.sup.2)
(kg/mm.sup.2)
pallet
__________________________________________________________________________
1 2 .times. 10.sup.5
gray
good 37.7 1966 good
2 8 .times. 10.sup.5
white
good 33.8 1850 good
3 8 .times. 10.sup.5
blue
good 29.9 1722 good
(1) 1 .times. 10.sup.5
black
good 34.7 1866 --
(2) 7 .times. 10.sup.8
gray
good 24.6 1480 --
(3) .sup. 5 .times. 10.sup.12
yellow
good 32.0 1948 --
(4) 2 .times. 10.sup.7
gray
poor 30.9 1771 --
(5) 2 .times. 10.sup.5
gray
good 30.6 1780 --
(6) 2 .times. 10.sup.5
gray
good 35.5 1945 poor
__________________________________________________________________________
It is noted from Table 1 that the light-colored electrically conductive
plastic composite materials in Examples 1 to 3 are superior to those in
Comparative Examples 1 to 6 in electrical conductivity (to prevent static
build-up) and color (to permit coloring). In addition, the former are
immune to delamination and deformation due to thermal deterioration and
have good mechanical properties. They meet all the requirements for
fastening pallets to be used to transfer printed circuit boards for their
automatic soldering.
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